ENH Normal Inverse Gaussian in scipy.stats

File distribution.py

According to the feedback I changed
1- removed spaces around ** (pep8 refuse that I remove \ for end of
lines)
2- removed m (or mu) as this is redundant with loc. Correct stats
accordingly
3- removed the redundant definition of logpdf and replace with
log(self._pdf(...))

Still open:
a) name (nig, norminv_gauss, norm_inv_gauss ?)
b) inclusion of init tests (I don't know which parameters I should
provide for for the distribution in the distribution list in
test_continuous_basic.py

General Problem
The computation of cdf and ppf is pretty slow in comparison to matlab or
R.
From my numerous tests, it seems that this has to do with the performance of the numerical integration